The invention relates to a corrosion resistant insecticidal composition for use in aerosol cans. In particular, it relates to an aqueous emulsified organic phosphorous pesticide composition which retards corrosion, especially pitting corrosion, in tin-plated steel aerosol cans.
Pressurized, water-based emulsified organophosphorous pesticides are known to be highly corrosive to tin-plated steel aerosol cans. In general, such aerosol cans are subject to at least three types of corrosion; general corrosion cathodic delamination of the can liner and pitting corrosion. The theory and mechanism of general corrosion and pitting corrosion is discussed in U.S. Pat. No. 4,240,925. To prevent conventional pitting corrosion, it has been proposed to employ a so-called anodic inhibitor to prevent the oxidation of iron and a cathodic inhibitor to prevent the corresponding reduction of oxygen to hydroxyl ion. The two half reactions, being inhibited, cannot then form a redox couple to promote rusting corrosion as follows: ##EQU1## It has been found that the use of organophosphorous insecticides, such as chlorpyrifos, (0, 0-diethyl-0-(3,5,6-trichloro-2-pyridyl) phosphorothioate, presents especially severe pitting corrosion problems to aerosol containers. Such containers are formed of rolled steel which is electrolytically coated with tin-plate and, optionally, with an organic film. Aqueous based, pressurized chlorpyrifos-containing compositions in aerosol cans exhibit shelf lives typically from three months to two years, depending upon the concentration of the pesticide. At lower concentrations of chlorpyrifos on the order of 0.5% by weight the critical shelf life can be on the order of 18 months, but at higher concentrations, the shelf life can be very short.
Recently, it has become highly desirable to provide concentrated chlorpyrifos-containing aerosol compositions. Such compositions are used in specifically adapted aerosol cans which are attached to the end of a garden hose. The concentrated solution is diluted with gardening water and immediately sprayed onto a desired application area in the proper dilution ratio to water.
Aqueous aerosol compositions, typically containing 2 to 5% organophosphorous insecticides have not, until now, been suitable for use in aerosol cans, since severe pitting corrosion occurs usually after only three months of storage. At higher temperatures such pitting corrosion occurs even more rapidly. Conventional corrosion inhibitors such as sodium benzoate, sodium nitrite, butoxyne, sodium citrate, sodium chromate and the like have not proved effective in preventing such pitting corrosion.
A corrosion inhibiting system has been proposed to prevent pitting corrosion in a pressurized rug cleaning composition. That composition, also known as Glory rug cleaner, is an aqueous pressurized solution (not emulsion) of various surfactants which is adapted to be sprayed onto a dirty rug. To stabilize the aerosol can containing the rug cleaning solution against conventional pitting corrosion, an inhibitor system of a (a) terpolymer of styrene-methyl methacrylate-maleic acid; (b) ammonia; (c) sodium benzoate and (d) disodium phosphate has been employed.
The mechanism of pitting corrosion in the pressurized rug cleaning solution is believed to be influenced by the presence of copper. Copper wire, employed in the process for forming the aerosol can, becomes incorporated in the can, itself. The copper and iron undergo a galvanic cell reaction during storage in the aqueous solution, such that the copper tends to plate out on the tin-plate, while the iron is oxidized, thus causing pitting corrosion. It is postulated that the terpolymer acts as a chelating agent for the copper ions to prevent the copper from plating out as copper metal. In the absence of the terpolymer the aerosol cans containing the rug cleaning solution rapidly puncture from pitting corrosion.
It is also believed that the ammonia, which is employed, in part, to stabilize the composition at a high pH, also acts to complex copper ions, thus also preventing copper metal from plating out. It is believed that the sodium benzoate inhibits the oxidation of iron, while the disodium phosphate acts to inhibit the reduction of dissolved oxygen to hydroxyl ion.
It is not possible to use the above-noted rug cleaner inhibitor system in an aqueous, emulsified, pressurized pesticide composition, since the terpolymer of styrene-methylmethacrylate-maleic acid breaks the emulsion. In addition, at the high alkaline pH of 9 typically produced by the ammonia, the emulsion is likewise broken.
Accordingly, it is an object of this invention to provide a pressurized, emulsified organophosphorous formulation having therein an effective corrosion inhibiting system to enable the composition to be stored in aerosol cans.